1. Field of the Invention
The present invention relates to a liquid crystal display device having a display mode using birefringence. In particular, the present invention relates to a liquid crystal display device having a display mode of forming a bend alignment in a liquid crystal material.
2. Description of the Related Art
Recently, a liquid crystal display device having an optically compensated bend (OCB) mode has attracted interests. In the liquid crystal display device, bend alignment is formed in a liquid crystal material, and a tilt angle of liquid crystal molecule is changed in the vicinity of each orientation film. In this way, retardation of a liquid crystal layer is changed.
According to the foregoing OCB mode, a bend alignment is essentially required as described above. However, it is difficult to stably obtain the bend alignment as described in the following.
In an initial state before power tune on, liquid crystal material forms a splay alignment. This is because the splay alignment is inherently stable as compared with the bend alignment. When the display device is started, an operation for making a transition from the splay alignment to the bend alignment is required. In order to develop the foregoing transition, energy more than state energy difference between bend and splay alignments needs to be given. Usually, voltage is applied to a liquid crystal cell, and thereby, given thereto in the form of electrostatic energy. According to the voltage application equivalent to energy difference, the progress of the foregoing transition is late; for this reason, very high voltage must be actually applied. The transition progress is easy to receive an influence by a shape of a substrate surface or electric field distribution. As a result, a non-transferred area remains in the liquid crystal layer.
JP-A2003-280036 (KOKAI) discloses the technique to solve the foregoing problem. According to the technique, an insert-die shaped bend pattern (hereinafter, referred to as transition nucleus forming portion) is provided around neighboring pixels. Potential difference is given between electrodes, and simultaneously, potential difference is given between counter electrodes. In this way, heavy distortion of liquid crystal alignment is generated in the thickness and inner-face direction of the liquid crystal cell. Thus, transition from splay alignment to bend alignment is made at high speed. The foregoing Publication JP-A2003-280036 (KOKAI) discloses a liquid crystal display device. In the liquid crystal display device, the transition nucleus forming portion is composed of a pixel electrode and a close electrode connected to a neighboring pixel electrode adjacent to the pixel electrode via a switching element. Potential difference equivalent to image signal amplitude is given between pixel electrodes, and simultaneously, potential difference is given between counter electrodes. In the liquid crystal display device, heavy liquid crystal alignment distortion is generated in the thickness direction and the in-plane direction of liquid crystal cell. A potential (voltage) change is prevented by capacitance coupling between the pixel electrode and a circumferential wiring to make a transition from a splay alignment to a bend alignment at high speed. According to the technique disclosed in the Publication JP-A2003-280036 (KOKAI), the transition nucleus forming portion is formed to be positioned just on a peripheral wiring electrode. Thus, numerical aperture and contrast can be kept higher.
In the liquid crystal display device disclosed in the Publication JP-A2003-280036 (KOKAI), different potential must be set to a counter electrode, individual pixel electrodes and proximity electrode at predetermined timing to realize high-speed transition. For this reason, a complicated drive control circuit is required. In order to configure a pixel using the proximity electrode, two switching elements are required per pixel. Thus, there is a problem that each configuration of an element structure and a drive circuit becomes complicated. Potential difference given to between neighboring pixels or between a pixel and the proximity electrode is set to a value equivalent to image signal amplitude (usually, 10V or less). If the foregoing potential difference is given, the field strength in the in-plane direction of the liquid crystal cell is short; as a result, a transition operation becomes unstable.